During diamond drilling for geotechnical application or mining exploration, an inner-tube is filled up with rock core. When the inner-tube is full, the operator needs to recover it. The inner-tube is linked with a head assembly or back end that permits to recover the inner-tube. An overshot is used like a fishing system to grab the head assembly. The overshot is in turn linked with a steel wire comprised in a wire line hoist system.
When an overshot and an inner tube are pulled out of a rod string for core recovery, there is a possibility that the overshot releases the spear head assembly by accident. If this happens, there is risk for workers injury and damage to the equipment.
The document U.S. Pat. No. 6,997,493 teaches a lockable overshot comprising an elongated body, lifting dogs and a locking sleeve. The ends of the lifting dogs are configured for latching a conventional spearhead point there between.
A locking sleeve is rotatable on the body between a locked state preventing ends of the lifting dogs from pivoting away from other to release a previously latched spearhead point, and an unlocked state where the locking sleeve allows the lifting dogs to move so that the ends can be pivoted away from each other to release a previously latched spearhead point. When the overshot goes out of the rod string, the operators have to stop, to lock, retrieve, unlock manually by rotating the locking sleeve into the unlocked position and manually pivot the two lifting dogs to separate the overshot from the spearhead assembly and get the core sample out. This is time consuming. Further, there is possibility operators choose not to lock the overshot at all. The operation can be difficult if dirt is present in the mechanism. At that time there is a risk for accident. The overshot also has the possibility to accidentally release the spearhead while still in the drill string, dropping the inner tube assembly to the bottom of the hole, causing damage to equipment.
The document U.S. Pat. No. 4,004,835 teaches an overshot comprising a tubular sleeve and a scissor like mechanism arranged moving as a unit. A spring bearing on the top end of the scissors tends to force them down into closing movement against an inner taper of the bottom ring of the tubular sleeve. The solution permits downward movement which releases the fishing neck. This avoids placing undue strain on the overshot.
Thus, there are needs to increase the safety and efficiency when working with an overshot. These needs cannot be fulfilled by overshot according to the above-mentioned prior art.